System of screen grid modulation



Jun@ 9 36- W. s. MARKS, JR

' SYSTEM oF SCREEN GRID MODULATION Filed July, 1951 MWLL/WM 7M/QMS, L/Q.

KEYR TIPE,

Patented June 9, i936 UNITED STATES i 2,043,255 PATENT oFFlcE SYSTEM OF SCREEN GRID MODULATION William S. Marks, Jr., Fort Monmouth, N. J. Application July 9, 1931, Serial No. 549,603

4 Claims. (O1. 179-171,)

(Granted under the act of March 3, 1883 as amended April 30, 1928; 370 0. G. 757) The invention described herein mayv be manufactured and used by or for the Government for governmental purposes, without the payment to me. of any royalty thereon.

This invention relates to a system of screen grid modulation and proposes a circuit arrangement for that purpose, which is adapted for impressing modulating potentials upon the anode screen grid electrode of tubes or electron dis-V charge devices having four electrodes or more.

An object of the` invention is to provide a system which may be used either for a telephone or telegraph transmitter;Y and which adapts itself particularly well to a combination radio telephone and telegraph set without sacrice of efficiency for either service. Substantially the saine apparatus can be used'for high speed keying in a telegraph system, as Well as for other modulating means such as voice or music in a telephone system. Since according to the present system little or no current is required tobe broken by the keying contacts, high speeds with positive action are assured.

An object of the4 invention is to` produce a system simple to install, and whichv lends itself for readyA application to existing radio trans-A mitters which use one or more screen grid tubes, or equivalents, without requiring radical Yalterations. Y

`Another important object of the invention is economy of operation. In the presentsystem the ratio of power required for modulation to the power to be modulated is very small, being of the order of 1/10 or less. The invention may be used in conjunction with any standard radio frequency oscillating; or amplifying system; and it permits relatively high powers, to be modulated with very little equipment, and without the necesa sity of building up large modulating voltages. Other objects will appear as the description pro-Y ceeds.

t Embodiments of the invention are illustrated in the accompanying drawing, in which:

Fig. l is a diagrammatic circuit arrangement showingl one form of the invention asv applied to a telephonetransmitter; and

l Fig. 2 shows the invention applied to a tele graph transmitter.

Referring rtoboth figures of the drawing, the

The tube here 9 is inserted in the plate circuit lead I0. The control grid connects with a radio frequencyv input supply,` which needs no .detailed description. The anode screen grid 5 is maintained at a positive potential relative to the filament or cathode 2', but preferably of less4 positive Value than that impressed upon the anode 4. It is understood that this tube may or may not be the final stage, as here represented; and only so much ofthe circuit diagram of the tube is` shown as is deemed necessary for an understanding of the operation, since such circuit arrangements are well known in the art. VTube II may be termed the modulator tube. This tube, which is placed in series with the screen grid circuit of tube I, is here shown as a three element tube comprising filament. I2, plate I3 and control grid I4, although'it is understood that other types of tubes may be substituted'. The plate lead I5 of this tube connects through radio frequency choke I6 with plate 4 of tube I, while the fllament lead I'I connects through resistance I8 with anode screen grid 5 of tube I. A by-pass condenser I9 is placed between the lament lead II and the positive side of the lament circuit of tube l. It will be noted that the plate filament circuit of tube I`I is placedk in series with the screen grid of tube I rather than with the second-` ary of a transformer. A lead 20 connects the plate circuit common to the tubes I and II to the positive electrode of a high voltage source of potential as at 2|. v

Referringto iig. 1, the modulator tube I I is iniiuenced by sound waves from microphone M, the modulated energy'being impressed upon control grid I4 of tube II in a conventional manner. A negative bias is maintained on grid I4 from the source generally denoted by -C. A blocking condenser is shown at 22.

' Referring tothe embodiment of the invention shown in Fig. 1, the mode of operation is outlined as follows: Modulation is effected by varying the screen grid potential of tube I at the desired frequency, The plate lament resistance of tube II changes'at the same'frequency as that impressed upon the microphone; this in turn changes the Screen-.grid current of tube I at the saine frequency. This varying screen grid current flowing through the plate-lament resistance and the' series resistance I8, causes a varying potential drop through such resistances, and this results in a varying screen grid potential, the variations of which are the Same as the frequency impressed upon the microphone. The plate current of tube I is changed at the same frequency as its. screen grid potential. which rement is shown as adapted for the work of high speed keying in a telegraph transmitter. This embodiment differs from that shown in Fig. 1, in the respect that another tube 24 is placed ahead of modulator tube II. nated as the keying tube and is here shown as a conventional three element tube having a filament 25, plate 26 and control grid 21. The plate circuit of tube 24 is fed from la regulated or variable high-voltage source 28 through variable resistance 29, having connection with the grid of tube II through lead 30. A voltage source 3| is inserted between the filament circuit of tube II and the battery 28. In the filament grid circuit of keying tube 24, instead of a microphone a key 32 is connected, or a tape transmitter may be substituted, if desired. In the circuit just mentioned a source of biasing voltage 33 is also inserted.

The mode of operation of the keying system of Fig. 2 is explained as follows: As in the instance of the embodiment shown in Fig. 1, the tube II functions as the modulator tube. Keying is effected by starting and stopping the iiow of screen grid current through this tube; that is to say, by opening and closing the screen grid circuit. Tube 24, hereinbefore designated as the keying tube, functions to alter the grid bias on the modulator tube I I in such a manner that when the key is open, sufl'icient negative bias is applied to the grid of tube II, so that no screen grid current flows through it; and when the key is closed a suiiiciently smaller value oi negative bias, or a positive bias, is applied to the grid of tube II to cause the proper screen grid current to flow to operate tube I.

In practice, it may be explained that when key 32 is open, the grid of tube 24 is free. The voltage of battery 28 is so regulated in respect to tube 24 that under the condition of free grid, plate current flows in said tube 24. This plate current flowing through resistance 29 puts a negative bias on the grid of tube II,A according to the adjustment of the variable contact or regulating means of said resistance and change of voltage of battery 3I. Adjustments are so made that when the key 32 is open, the negative bias placed on the grid of tube II is suflicient to prevent the screen gri-d current of tube I from flowing through tube II. When the key 32 is closed, the negative potential of battery 33 is applied to the grid of tube 24, which cuts off its plate current. When no plate current ows, there is no IR drop through resistance 29, thus a smaller negative potential, or a positive potential, as previously indicated, is placed upon the grid of tube II, which causes the screen grid current of tube I to ow through tube II and permits tube I to function properly.

The choice of tubes having suitable characteristics, and the regulation of voltages for successful operation of such a system as herein disclosed, will be obvious to those skilled in the art. It will be noted that when the necessary adjustments have been made, no current will be broken by the key contacts, which makes possible very high keying speeds.

The present system is readily applicable to circumstances requiring a combined telegraph and telephone transmitter. Obviously, the change from telegraph to phone, and vice versa, may be made with no sacrifice in elliciency. The

Tube 24 may be desig-Y adjustment of the transmitter for best modulation is taken care of in the modulator unit only. Such a system adapts itself to transmitters which use rectified grid current for bias control, thus making it possible to obtain high modulation with minimum distortion.

It Will be obvious that stages either of radio or audio amplification may be employed as may be found necessary or desirable; for example, audio amplification may be interposed between the microphone M and the grid filament circuit of tube I I, as shown in Fig. 1, in accordance with the voltages required on the grid of said tube I I and to conform to the type of microphone and microphone transformer used.

The invention has been described in connection with certain preferred embodiments, but it is understood that it is capable of changes and modifications within the scope of the appended claims.

What is claimed is:-

l. A system of screen grid modulation, comprising in a transmitter circuit, an electron tube to be modulated, having anode screening grid means; means for maintaining a positive bias on said anode screening grid means; a modulator tube operatively coupled in series with said iirst named tube; an auxiliary tube operatively coupled with the said modulator tube; and means for impressing modulating frequencies upon said anode screening grid means through said auxiliary tube and said modulator tube, including means to` start and stop the flow of screen grid current through said modulator tube.

2. A system of screen grid modulation, comprising in a transmitter circuit, an electron tube having an anode and anode screening grid means; a screen grid circuit and means in said circuit for maintaining a positive potential on said anode screening grid means, but of less positive value than on said anode; a modulator tube in said screen grid circuit operatively coupled in series with said first named tube; and modulating means to start and stop the flow of screen grid current through the modulator tube, said means including an auxiliary tube coupled ahead of the modulator tube.

3. A system according to the preceding claim in which said modulating means to start and stop the flow of screen grid current through the modulator tube comprises keying means; variable resistance means; a variable source of potential; and an auxiliary tube, all interposed between the keying means and said modulator tube.

4. system of screen grid modulation, comprising in a transmitter circuit an electron tube to be modulated, said tube having anode screening grid elements; a modulator tube in said circuit operatively coupled in series with the first named tube; a keying control tube in said circuit operatively coupled with said modulator tube; means for impressing modulating variations upon said anode screening grid elements through said keying control tube and said modulator tube, said means comprising a key and grid biasing means operative through said keying control tube to start and stop the flow of screen grid current through said modulator tube as said key is closed and opened.

WILLIAM S. MARKS, JR. 

